Ulsan National Institute of Science and Technology (UNIST): Multicolor 4D Printing of Shape-Memory Polymers

Researchers from Ulsan National Institute of Science and Technology (UNIST) take 3D printing to the next level, releasing findings from their study in the recently published ‘Multicolor 4D printing of shape-memory polymers for light-induced selective heating and remote actuation.’

While the miracles of 3D printing continue to abound, 4D printing allows users to work with materials that respond to their environment, deforming accordingly—and then reverting to their initial, natural shapes. In this study, the authors 4D print multicolor shape-memory polymers (SMPs) and demonstrate how light absorption and subsequent heating of the material cause remote actuation.

Previous studies have tended to focus on how SMPs deform in the presence of heat or moisture, but here light is used as a powerful force in causing stimuli-responsive changes. Selective heating was allowed due to choices in color of light, also resulting in color-dependent structural transformations.

“4D printing can allow the complex geometries of multicolor composites with predesigned responses,” stated the authors. “In addition, SMPs can be reused multiple times by conducting thermomechanical programming again. Therefore, multicolor 4D printing of SMPs can offer unique merits for light-induced structural changes and remote actuation.”

For the study, the authors created a light-activating structure measuring L = 40 mm, w = 5.5 mm, t = 2 mm, a = 0.4 mm, and made of three materials:

• Yellow (Veroyellow)
• Blue (Verocyan)
• Sky-blue matrix (Tango +)

A 3D printed light-activating structure. (a) Schematic for the multicolor SMP structure. (b) Side view of the structure. (c) Thermomechanical programming and bending behavior (the dotted line in the figure is an eye guide).

Light was able to reach both the yellow and blue fibers due to strategic positioning of the fibers. After 3D printing and post-processing, the structure was bent downward, reverting to its initial shape after being exposed to blue light. Through continued experimentation with color dependent selective heating, the researchers realized that they could manipulate actuation through sequences of light.

Bending behavior of the multicolor sample. A thermomechanically programmed structure bends to a n-shape under red illumination. After bending, the structure can recover to an initial flat state with blue illumination. In case of illuminating blue light first, the structure bends to a U-shape. It can also recover to the initial state with red-light illumination. (a) is the schematic for dual-step actuation, while (b) is the experimental result.

“Applying red light later caused the entire structure to retain its initial flat state. However, when the structure was heated in hot water (instead of selective heating with colored light), the change in shape of our sample was insignificant (data is not shown here). In the hot water, both blue and yellow SMPs recovered at the same rate, and the entire structure shrank to its original length but remained flat (i.e., no shape change occurred),” stated the researchers.

“The rise in temperature due to direct blue-light absorption was significantly smaller than that due to heat transfer. Thus, it shows that the dominant factor causing the temperature increase in the blue fibers at the lower layer was the heat transferred from the yellow fibers at the top layer.”

(a) The measured temperature change of the multicolor structure. The yellow solid line is the temperature of yellow SMP fibers, whereas the blue dashed line indicates the temperature of blue SMP fibers obtained from heat transfer simulations. The red solid line is the measured temperature of blue SMP fibers in a control sample that contains blue SMP fibers only. (b) Results of solid-mechanics simulations. The color bar indicates the total displacement measured from the bottom plane.

Bent structures reverted to a flat shape due to heat being transferred while illuminated; however, this type of heat transfer occurred after the light was turned off also, evidenced by slight relaxing afterward. The researchers attributed this action to residual heat in the structure, allowing for relaxation in the form. To prevent this, they considered adding a thermal insulating layer.

Their final sample included a hinged structure meant for experimentation with multistep actuation. The team used colored SMP fibers in the hinges, manipulating deformation through colors. Rapid transformation occurred as they focused the LED light onto the structure with a focal lens.

Multicolor hinged structure for multistep actuation. (a) Schematic for the multicolor hinged structure. (b) Example of multistep actuation. This hinged structure can transform into different 3D shapes depending on the color of light and duration of illumination.

“SMPs can be reused by conducting thermomechanical programming again and their response temperatures can be adjusted via material synthesis or by dynamic mixing during 3D printing. Moreover, 4D printing can enable the fabrication of complex, multicolor geometries for tailored responses. Therefore, multicolor 4D printing of SMP composites have unique merits for light-induced structural changes and remote actuation,” concluded the researchers.

(a) Storage modulus and (b) Loss tangent obtained with dynamic mechanical analysis (DMA) measurements.

4D printing continues to gain traction with users around the world, particularly researchers, investigating nanoscale 4D printing, customized printing for metastructures, 4D printing in optics, and more.